Abstract: A systematic study concerning the effect of strain rate on deformation mechanism of UHMWPE fibers at high temperature over a wide range of strain rates is carried out by methods of simultaneous stress-strain curves, in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The mechanical behavior shows a transformation from similar brittle fracture behavior to typical ductile fracture behavior. And the dynamic mechanical property (DMTA) reveals the existence of mechanically active relaxation process. Moreover, characteristics of structural evolutions based on analysis of WAXS and SAXS patterns show dramatic difference especially when reaches specific stretching strain rate. Interestingly, the turning point of strain rate can coincidently be distinguished from DMTA characterizations. According to the compatible results above, we believe that the key point to understand strain rate dependence on deformation process lies in the matching degree between chains mobility in crystaline region or amorphous region and strain rate. A diagrammatic graph connecting non-linear mechanical behavior and structural evolutions at different strain rates, which gives a hint of the origin of morphological transformation diversity, is successfully established.